Speedometer not working, cruise okay, stepper okay, could the issue be a bad ECM?

BUMPY RHODES

MEMBER
2003 CHEVROLET SILVERADO
5.3L
V8
4WD
AUTOMATIC
160,000 MILES

The speedometer and odometer stopped working a few months ago. Cruise control works perfectly, the engine runs great, no codes, no CEL. Replaced speed sensor, no change. Installed a different gauge cluster, no change. Had gauge cluster professionally rebuilt, no change, speedo doesn't move from zero.

My guess is broken wire (can't find any) or bad ECU? Your opinion would be greatly appreciated, thanks!

6 Replies

KEN L

MASTER CERTIFIED MECHANIC
46,719 POSTS

Yep, it sounds like a bad ECU because the cruise control gets its data from the VSS via the ECU so it must not be parsing the data to the speedometer. To be sure let's probe the dr-grn/wht wire at the cluster and have the voltmeter on OHMS, ground the lack lead and take it for drive, if the resistance does not change the ECU is bad. Also, check the cluster fuses as well, here are the fuses that need to be checked and two guides to help us see what's going on:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

and

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Here is how to change out the PCM in case you need it, you can get a preprogrammed unit by searching Google or Ebay. Check out the images (below). Please let us know what happens.

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Monday, June 24th, 2024 AT 11:19 AM

BUMPY RHODES

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4 POSTS

Thanks. Fuses checked, okay. There are 3 identical green wires with white stripe, all indicated OPEN circuit at all times: off, on, running, driving 15mph. I guess I'm replacing the PCM. I see remanufactured PCM''s on eBay for $100. Is a special scan tool required? Thanks.

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Monday, June 24th, 2024 AT 2:18 PM

KEN L

MASTER CERTIFIED MECHANIC
46,719 POSTS

They should be pre-programmed so no scanner required. But first let's check the connection between the PCM and cluster, here is the complete wiring diagrams for the system so you can see how it works, use this guide to help test.

https://www.2carpros.com/articles/how-to-check-wiring

pin #50 in the C2 connector for the pcm (dr grn/wht) and pin #A4 in the cluster. Check out the images (below). Let us know how it goes.

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Tuesday, June 25th, 2024 AT 10:06 AM

BUMPY RHODES

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4 POSTS

Success! I really didn’t want to trace wires but you guys spent the time and effort providing accurate and detailed info so I felt compelled. Good thing, a new PCM would not have fixed anything! There was no continuity in the wire so I started trying to trace it. The bundle went under the fuse box so I removed the shroud and right on top the green and white was gnawed in two! (Near the top of the photo.) Mice or chipmunks were hungry I guess! I installed a jumper and the speedo and odo worked! I am extremely delighted and impressed!

I made a donation on GFM. I am shocked how well this turned out! Thanks!

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